A textile composite is a combination of a resin system with textile fibers (fibers, yarns or labrican, The textile component provides the tensile strength and rigidity. This is due to the molecular orientation of the fiber resulting in a strong and stiff element in the fiber direction. High performance fibers posses high strength or high modulus properties. The most important high performance fibers are composed of glass, graphite, aramid, polyethylene, boron, ceramic or steel. The resin or the matrix component holds the textile reinforcement in a prescribed suspension, provides rigidity and helps to distribute external loads on the material through the fibers. The matrix also protects the fibers from external injury and environmental effects (corrosion, radiation, etc.).
Composite materials produced from high performance fibers are called high performance composites. Such materials are becoming increasingly important in aerospace and aircraft application due to their high-strength and stiffness-to-weight ratios,
Most advanced composites are formed by stacking (laminating) layers of fabric and then bonding them together to one solid structure, The layers may consist of fabrics, tapes, mats or unidirectional fibers laid in several directions. The weakness of the laminated structure is in its tendency to delaminate.
In order to overcome the weakness of delamination, It Is necessary to reinforce the composite structure in three dimensions. One way of achieving it is by using tile non-woven technique. This technique involves "felting" of short-length fibers so that the fibers interlace in three dimensions. The interlacing can be performed by two dimensional punching of short fibers web with needles that orient some of the fibers in the third dimension. When using very short fibers, it is possible to blend the fibers with the resin and process it in conventional polymeric machines such as extruder or injection molder.
The major limitations of the composites made in this way from short fibers are the lack of control on the fiber orientation and the mechanical inferiority of short fibers relative to continuous filaments. Non-woven structures offer limited design or shaping capability but are simple and cheap to produce.
Three dimensional (3-D) fabrics for structural composites are fully integrated continuous fiber assemblies having fiber orientation in the X, Y and Z dimension. Composites made from 3-D fabrics are superior in withstanding multidirectional mechanical stresses and thermal stresses. The three basic classes of integrating fibers, in yarns form, to 3-D structures are braiding, knitting and weaving.
In braiding, fabric is constructed by intertwining or orthogonal interlacing of two or more yarn systems to form an integral structure. The yarns are fed continuously from coned packages to the braiding machine. A 3-D braiding system can produce thin and thick structures in a wide variety of complex shapes. Fiber orientation can be chosen and 0.degree. longitudinal reinforcement can be added, but a true three mutually perpendicular axes of straight yarn segments cannot be achieved.
Knitted fabrics are interlooped structures. The knitting loops are produced either by feeding the yarn in the cross machine direction (weft knit) or along the machine direction (warp knit). The latter is more suitable for 3-D composite reinforcement. Multiaxial warp knit structures consist of warp yarns at 0.degree., warp yarns at 90.degree. and other yarns at an angle .+-.0 to the warp yarn direction. These yarns are held together by a chain of tricot stitches. The knitting process involves bending of the yarn in the knitting needle and sometimes piercing of the needles through the yarn layers. Both operations are not recommended for brittle yarns such as glass, boron and graphite.
3-D woven fabrics can be produced by conventional weaving, using multiple warp. The number of layers (warps) used in this method is limited by the friction resulting from the shedding motion and beat-up motion. Using this method, various yarn architectures can be woven. Orthogonal 3-D weaving can be fabricated by maintaining one stationary axis and inserting the yarns orthogonal to the axial yarn's system In an alternating manner. The same method is used for the formation of a tubular 3-D fabric. The advantage of the orthogonal weaving is in the linear yarn reinforcement in all directions. Bending or kinking the reinforcing yarns can cause deterioration in the mechanical properties of the composite material. However, the insertion of the orthogonal yarns through the yarns of the stationary axis, may produce technological problems and even damage to the stationary yarns.
Such a method of weaving is described in the U.S. Pat. No. 3,834,424 to Fukuta et al. King in U.S. Pat. No. 4,001,478 disclosed another method to form a 3-D structure of rectangular cross-section. U.S. Pat. No. 5,085,252 by Mohamed et al. describes a method of forming variable cross-section shaped 3-D fabrics. These patents and others emplby various techniques of inserting weft yarns through a planar array of warp yarns. When the array consists of a population of highly densed warp yarns, which is the case when high volume ratio of fibers is required, the weft Insertion operation can cause injury to the warp yarns.